Beta-isophorone is of great industrial interest for the synthesis of important classes of compounds. In particular, beta-isophorone is required as an intermediate for ketoisophorone (2,6,6-trimethylcyclohex-2-ene-1,4-dione) which, in turn, is used for the manufacture of vitamin E, various carotinoids, odorants and other natural products.
The manufacture of isophrone takes place by the water-cleaving trimerization of acetone. However, in this case the alpha isomer is the main product formed, in yields above 90%. The thermodynamic equilibrium of the alpha-isophorone/beta-isophorone system lies on the alpha-isophorone side, because by displacing the double bond, a lower energy conjugated system must be converted into a higher energy non-conjugated system. Moreover, the adjustment of this equilibrium takes place slowly. This means that the conversion of alpha-isophorone into beta-isophorone is associated with difficulties.
Of particular interest, therefore, is the isomerization of alpha-isophorone to beta-isophorone in high yield and in an economical manner.
Several processes for the isomerization of alpha-isophorone to beta-isophorone have already been described. The isomerization can be carried out, e.g., by reacting alpha-isophorone with molar amounts of methyl magnesium iodide with the addition of iron(III) chloride, subsequent hydrolysis and distillative working-up [A. Heymes et al., Recherches 1971, 18, 104-108].
The isomerization can also be effected by boiling alpha-isophorone for several hours with triethanolamine, subsequent fractionation and washing the distillate with tartaric acid and sodium chloride solution [DE-OS 2457157].
The catalytic isomerization of alpha-isophorone with acids which dissociate little [U.S. Pat. No. 4,005,145] is a further route to beta-isophorone. In this process the beta-isophorone formed is removed by distillation from the equilibrium. The content of beta-isophorone in the distillation still amounts to 1-2%.
Catalytic isomerization may also be carried out using acetylacetonates or metals of groups IVB, VB, VIB, VIIB and VIIIB of the periodic system as well as aluminium [EP 312 735].
However, all of the known processes have disadvantages, namely:
The adjustment of the equilibrium is a relatively slow process even with the best of the hitherto known catalysts. The weight/time yields are correspondingly low. PA1 A re-isomerization takes place at a reaction temperature above 200.degree. C. in the distillative separation of beta-isophorone from alpha-isophorone. In order to obtain pure beta-isophorone a second distillation under reduced pressure is required. PA1 The energy expenditure for the distillative separation of the alpha-isophorone/beta-isophorone mixture is high, since the content of beta-isophorone is very small.